linux-kernel - Re: PROBLEM: AST2500 BMC video output disabled by reboot (regression)

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Message-ID: <qjyncqymjja57h2wxxv3ebuqpip5qu7yjalccons5xmtzfw5h5@m3u4rvbqzcxt>
Date: Fri, 3 Oct 2025 20:31:51 -0400
From: Nick Bowler <nbowler@...conx.ca>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Doug Anderson <dianders@...omium.org>, linux-kernel@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, regressions@...ts.linux.dev
Subject: Re: PROBLEM: AST2500 BMC video output disabled by reboot (regression)

On Wed, Oct 01, 2025 at 09:26:28AM +0200, Thomas Zimmermann wrote:
> Am 18.09.25 um 13:17 schrieb Thomas Zimmermann:
> > Am 18.09.25 um 04:04 schrieb Nick Bowler:
> > > On Wed, Sep 17, 2025 at 11:14:45AM -0400, Nick Bowler wrote:
> > > > On Fri, Aug 29, 2025 at 03:07:14PM +0200, Thomas Zimmermann wrote:
> > > > > The ast driver doesn't do much during shutdown. Could you
> > > > > please out-comment the lines at either [2] xor [3] and report
> > > > > on either effect?
[...]
> > > > > [2] https://elixir.bootlin.com/linux/v6.16.3/source/drivers/gpu/drm/ast/ast_mode.c#L835
> > > > > [3] https://elixir.bootlin.com/linux/v6.16.3/source/drivers/gpu/drm/ast/ast_mode.c#L839
[...]
> > >    - Deleting [3] (only) appears sufficient to make things work again,
> > >      that is, deleting the following line in ast_mode.c:
> > > 
> > >        ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0xb6, 0xfc, vgacrb6);
> > 
> > Please test if the attached patch fixes the problem for you.
> 
> Have you been able to test the patch?

In the normal scenario where everything is working and I reboot, then
the display remains on for the firmware (this would seem to be an
improvement).

But it introduces a new problem: the screen no longer turns back on if
I boot the patched kernel from the "display off" state.  The unpatched
6.17 kernel would at least turn the display back on from this.
Furthermore, rebooting from this state keeps the display off.

The earlier change [3] above has no such problems.

Thanks,
  Nick